Actin polymerization accompanies receptor-mediated responses in many cell types and is correlated with motility-related processes. T lymphocytes have a lateral redistribution of surface receptors with concomitant aggregation of actin-myosin in cytoplasmic subcaps. Using purified resting T cells from 4-6 mo, 15-17 mo, and 24-25 mo old C57BL/6 mice, F-actin was quantitiated with Bodipy conjugated to phallacidin followed by flow cytometry. During the first 5 min of activation by 7.5 microgram/ml Con A, F-actin in cells from young mice depolymerized to 78% of resting levels, reaching its lowest point in 2 min. and then rapid polymerization ensued. In 15-17 mo old mice, a similar but blunted pattern was observed for 45 sec. followed by insignificant polymerization. In contrast, no depolymerization nor polymerization phases occurred in cells from aged mice. To circumvent early activation events which generate second messengers, phorbol 12 myristate 13-acetate (PMA) amd ionomycin were used to activate protein kinase C (PKC) and to increase cytosolic calcium respectively. Both PMA and PMA plus ionomycin caused F actin polymerization in lymphocytes from young and aged mice, but to only 57% of that in Con A - stimulated cells from young mice. We suggest that 1) receptor occupancy and activation of the phosphoinositide cycle are necessary for complete polymerization responses in lymphocytes from young mice, and 2) the age-related impairment of actin filament assembly is likely due to early signalling events prior to activation of PKC.